(1) Field of the Invention
The invention relates to a method of preventing etching damage to a passivation layer in the fabrication of integrated circuits, and more particularly, to a method of preventing etching damage to a passivation layer by crosslinking a negative resist in the manufacture of integrated circuits.
(2) Description of the Prior Art
The manufacture of integrated circuit devices has progressed to the point where half micron and sub-half micron feature sizes are common. In this technology, the top metal spacing becomes narrow enough so that keyholes are formed within the silicon oxide/silicon nitride layers covering the metal lines for certain geometrical patterns, such as at the turning points of a group of parallel metal lines.
For example, FIG. 1 illustrates a semiconductor substrate 10. Layer 14 contains various semiconductor device structures and insulating layers, not shown. The topmost metal layer 20 is shown overlying layer 14. Typically, the metal layer is passivated by first depositing a layer of silicon oxide 22 by plasma enhanced chemical vapor deposition (PECVD). Then, a silicon nitride layer 24 is deposited also by PECVD. Because the gap between the metal lines is so small, a keyhole 25 can form within the gap.
After the passivation layer has been deposited, a layer of photoresist is coated over the passivation layer and the layer is patterned as desired. However, when the photoresist 30 is coated over a layer containing keyholes, the resist will flow into the keyholes resulting in a thinner resist layer in these areas, as shown in FIG. 2. During the plasma etching process, a portion of the photoresist mask will be eroded away. Because of the thinner resist in the areas of the keyholes, the passivation layer in those areas may be exposed by the eroding of the photoresist causing damage in the device areas.
After the etching step, the photoresist mask is stripped. This is typically done by a wet strip followed by O.sub.2 plasma ashing. As illustrated in FIG. 3, the photoresist 31 within the keyhole may harden so that it cannot be removed by the photoresist strip. Wet chemicals 33 from the wet strip may be trapped around the hardened photoresist 31. Hard baking both before and during the plasma etch and O.sub.2 plasma etching all are performed at high temperatures which can cause the resist to harden.
The wafer is then annealed in hydrogen and nitrogen at between about 400 to 450.degree. C. This alloy process provides H.sub.2 at a relatively high temperature to react with the silicon dangling bonds to stabilize the SiO.sub.2 --Si interface. The alloy process is used to release trapped interface charges from the plasma processes, including etching, depositing, and ashing. During this annealing, the hardened photoresist 31 may be extruded from the keyhole. The wet chemicals 33 previously trapped by the photoresist 31 would evaporate and cause defects to the device.
The most common solution to this problem is to planarize the passivation layer by covering it with a spin-on-glass layer or a silicon oxide layer deposited by subatmospheric chemical vapor deposition (SACVD) and then etching back. Then, the photoresist is coated onto the planarized layer. However, this method changes the passivation film structure requiring further device reliability qualification and dramatically increased production costs.
U.S. Pat. No. 5,494,853 to Lur discloses a method of metal patterning, including metal islands and dummy vias, that will prevent openings to tunnels and holes within a passivation layer and thereby prevent a photoresist coating from sinking into the holes. U.S. Pat. No. 5,007,234 to Scoopo et al teaches a planarization process utilizing three resist layers. U.S. Pat. No. 4,794,021 to Potter discloses a photoresist reflow technique to form a uniform thick photoresist coating. A much less complicated and costly method for solving the resist thinning problem over keyholes is desired.